Safe operation of commercial and military aircraft during icing conditions requires the removal of ice and frozen deposits from critical surfaces prior to takeoff. Military aircraft especially must be capable of safely conducting operations in adverse winter weather if required by the mission. Mission specific readiness requirements and long intervals between successive flights expose military aircraft to ground icing weather conditions for much longer periods of time than commercial aircraft, allowing the buildup of excessive ice and snow contamination on aircraft surfaces. In addition, extensive pre-flight checks on sophisticated aircraft subsystems may require aircraft to remain on the ground, exposed to icing conditions for relatively long periods of time after being deiced (Arthur, D., Comments from a presentation on operational requirements for deicing/anti-icing fluids, 317 AS Charleston AFB Reserves, Air Force Deicing Technology Crossfeed, Washington, D.C., August, 1996). Thus, the capability to rapidly remove and prevent the reformation of ice is essential to the readiness and safety of military air operations.
For the past five decades, ice removal (deicing) of commercial and military aircraft have been accomplished by applying heated solutions of water and ethylene glycol (EG) or propylene glycol (PG) freezing point depressants (FPD). These processes use very large quantities of deicing fluids to effectively remove ice from critical aircraft surfaces and prevent its reformation. For example, an aircraft the size of a Boeing 727 commercial airliner can use between 200 and 1000 gallons of deicing fluid for a single pre-takeoff deicing procedure. According to data collected by the USAF Logistics Management Agency, the US Air Force alone uses approximately 1.2 million gallons of deicing fluid per year worldwide. Recently, commercial airlines have begun to use anti-icing fluids. These fluids are characterized by their ability to prevent ice formation on the aircraft surface for extended periods of time while the aircraft is awaiting takeoff during episodes of freezing or frozen precipitation. This extended period of ice protection, known as “holdover time”, is often obtained by adding thickeners or surfactants to the fluid. About 50 gallons of anti-icing fluid are used per aircraft.
During deicing operations, thousands of gallons of aircraft deicing fluids flow off the aircraft and into the environment through storm sewers. Approximately 90 percent of deicing fluid and 15 percent of anti-icing fluid run off the aircraft in the application area. These sporadic releases of deicing fluids have caused a number of reported incidents of environmental damage near commercial airports and military installations throughout the world. For example, in 1991, the USAF decided to ban further purchase of EG-based deicing fluids because of an incident of groundwater contamination at Griffiss AFB (New York). Ethylene glycol is a listed hazardous air pollutant (HAP) and a listed groundwater pollutant in several states because of its toxicity and faces regulatory burdens in use, handling, and transportation (Oswalt, B. E., et al., “Use of Hazardous Materials for Aircraft Deicing,” Report No. LM 930081, Air Force Logistics Management Agency, May, 1993). Propylene glycol, on the other hand, is an FDA approved food additive and has been shown to be virtually non-toxic to humans and terrestrial life. Glycol-based freezing point depressants (PEG and PG) have relatively low aquatic toxicity. However, additives such as corrosion inhibitors, surfactants (wetting agents) and thickening agents used in anti-icers increase the aquatic toxicity by orders of magnitude over that of pure EG or PG.
When a substance experiences aerobic biodegradation in water, it utilizes dissolved oxygen from the water. The amount of oxygen required to degrade a unit amount of substance over a specified time period is called the biochemical oxygen demand (BOD). Typically, BOD is measured over a 5 day, 20 day, or 28 day period. If a substance is allowed to biodegrade over a sufficient period of time that it achieves its ultimate state of biodegradation, it will exert its ultimate or total BOD. Substances with a high BOD may deplete the dissolved oxygen in a receiving body of water such that the remaining dissolved oxygen is insufficient to support aquatic life. Typical environmental regulatory guidelines call for a minimum dissolved oxygen content in receiving waters of 5 mg/L. The five-day biochemical oxygen demand (BOD) for glycol-based freezing point depressants and commercially available deicing and anti-icing fluids range from about 400 to 800 g/kg, much greater than those found in typical raw municipal sewage (D'Itri, F. M., “Deicers in Airport Stormwater Runoff,” in Chemical Deicers and the Environment, p. 327, Lewis Publishers, 1991).
As a result of these adverse environmental impacts, the US Environmental Protection Agency (US EPA), and state and local environmental agencies have promulgated increasingly stringent regulations affecting aircraft deicing fluid runoff. The Clean Water Act Amendment of 1987 requires airports to obtain a National Pollutant Discharge Elimination System (NPDES) permit to discharge deicing fluids into the storm sewer. These permits limit the concentration and nature of pollutants being discharged into the storm sewer and require implementation of a pollution prevention plan to reduce the amount of pollutants released to the environment.
In order to comply with environmental regulations, a number of commercial airports have elected to install systems designed to capture and control runoff from deicing operations. Once the runoff is captured, it must be contained for treatment. Biological treatment of runoff is increasingly gaining favor. This method contains the deicing fluid runoff in holding tanks or lagoons until either the glycol is sufficiently biodegraded or the water can be safely metered out to a publicly owned treatment works (POTW) or an on-site bioreactor. Treatment costs at a POTW are often related to the BOD content of the waste stream. In addition, waste streams with high ultimate BODs must be metered into the POTW slowly in order to avoid creating anaerobic conditions in the treatment system. Due to the high ultimate BOD of deicing and anti-icing fluids, treatment costs are quite high and put a tremendous demand on the capacity of a biotreatment facility. The cost of environmental compliance has greatly increased the life cycle cost of deicing fluids over the past decade. In fact, if the cost of purchase, handling and disposal of deicing fluids is considered, one deicing expert estimates the total life cycle cost of a gallon of glycol based deicer at $17 to 21 per gallon (Williams, L., “Comments from a presentation on forced air deicing equipment,” FMC Corporation, Air Force Deicing Technology Crossfeed, Washington, D.C., August, 1996; Lamb, C. B. and Jenkins, G. F., “BOD of Synthetic Organic Chemicals,” Proc. 7th Ind. Waste Conf., 79, pp. 326-339, 1952).
Fluids containing glycol freezing point depressants have typically been used to remove ice or prevent ice from forming on aircraft prior to takeoff. Commercially available deicing fluids (SAE Type I) largely consist of combinations of ethylene glycol, diethylene glycol or propylene glycol freezing point depressants with water. These fluids may also contain surfactants, corrosion inhibitors, dyes, pH buffers, and chelating agents to produce required properties. Commercially available anti-icing fluids (SAE Types II, III, and IV) contain similar ingredients plus a thickener or a combination of surfactants to impart non-Newtonian theological properties.
It would be desirable to have deicing and/or anti-icing compositions that are biodegradable and non-toxic to animals and the environment. It would also be useful to be able to control or tailor the biodegradation rate, or short term (five-day) and ultimate BOD, of these compositions, depending on the environmental and hydrogeological conditions that prevail in a given body of receiving water. For example, if an airport's deicing fluid runoff enters a small, slow-moving body of water with limited ability to replenish dissolved oxygen from the atmosphere, a rapidly degrading substance with a high short term (5-day) BOD will deplete dissolved oxygen levels more than a slowly degrading substance with a lower short term (5-day) BOD. If dissolved oxygen depletion is severe enough, this condition may lead to the death of aquatic organisms in the receiving water body. On the other hand, if the fluid runoff is captured and subsequently biotreated, a slowly degrading substance with a lower short term (5-day) BOD will require longer treatment times, necessitating a larger treatment facility volume compared to a rapidly degrading fluid.